Methods and systems for operating HVAC systems in low load conditions

Abstract

A system comprising a compressor, a first valve coupled to the compressor and coupled to a first coil, a first expansion valve coupled to the first coil, a second coil, and a second expansion valve. The second expansion valve coupled to a third coil, a second valve coupled to the compressor and the third coil. A controller operable to operate the first valve, the first expansion valve, the second expansion valve, and the second valve. The second coil is coupled to the compressor and the refrigerant flows from the second coil to the compressor.

The invention claimed is: 
     
       1. A method of operating a variable refrigerant flow (VRF) system, the method comprising:
 determining that a cooling demand of the VRF system is below a threshold cooling demand; 
 in response to determining that the cooling demand is below the threshold cooling demand, generating an artificial cooling load, wherein generating the artificial load comprises: 
 operating a first valve to direct refrigerant discharged from a compressor only into a first coil; 
 operating a first expansion valve to direct an amount of the refrigerant received from the first coil to a second expansion valve; 
 operating the second expansion valve to direct the amount of the refrigerant to a third coil; and 
 operating a second valve to direct the amount of refrigerant to flow from the third coil to the compressor. 
 
     
     
       2. The method of  claim 1 , wherein the first and third coils are placed outside a building. 
     
     
       3. The method of  claim 1 , further comprising regulating a speed of the compressor by using a variable speed drive coupled to the compressor. 
     
     
       4. The method of  claim 1 , further comprising operating one or more fans to blow ambient air across the first coil or the third coil and regulating a speed of the one or more fans based in part on the cooling demand. 
     
     
       5. The method of  claim 1 , further comprising:
 causing the first coil to function as a condenser; and 
 causing the third coil to function as an evaporator. 
 
     
     
       6. The method of  claim 1 , further comprising:
 determining that the cooling demand ceases to be below the threshold cooling demand; and 
 in response to determining that the cooling demand ceases to be below the threshold cooling demand, de-energizing the VRF system, thereby ceasing cooling operation of the VRF system. 
 
     
     
       7. The method of  claim 1 , further comprising, in response to detecting a change in the cooling demand:
 varying a degree of openness of one or both of the first expansion valve and the second expansion valve; and 
 varying a speed of a fan configured to direct air across the third coil, thereby adjusting heat transfer between the third coil and the air and a magnitude of the artificial load. 
 
     
     
       8. The method of  claim 1 , wherein generating the artificial cooling demand further comprises;
 directing a second amount of the refrigerant received from the first coil to a second coil; and 
 directing the second amount of the refrigerant from the second coil to the compressor. 
 
     
     
       9. A method of operating a variable refrigerant flow (VRF) system, the method comprising:
 determining that a cooling demand of the VRF system is below a threshold cooling demand; 
 in response to determining that the cooling demand is below the threshold cooling demand, generating an artificial cooling load, wherein generating the artificial load comprises:
 operating a first valve to direct refrigerant discharged from a compressor only into a first coil; 
 operating a first expansion valve to direct an amount of the refrigerant received from the first coil to a second coil, wherein the second coil is coupled to the compressor and at least a portion of the amount of the refrigerant received by the second coil flows from the second coil to the compressor. 
 
 
     
     
       10. The method of  claim 9 , wherein the second coil is placed inside a building and the first coil is placed outside the building. 
     
     
       11. The method of  claim 9 , further comprising regulating a speed of the compressor by using a variable speed drive coupled to the compressor. 
     
     
       12. The method of  claim 9 , further comprising operating one or more fans to blow ambient air across the first coil and regulating a speed of the one or more fans based in part on the cooling demand. 
     
     
       13. The method of  claim 9 , further comprising:
 causing the first coil to function as a condenser; and 
 causing the second coil to function as an evaporator. 
 
     
     
       14. The method of  claim 9 , further comprising:
 determining that the cooling demand ceases to be below the threshold cooling demand; and 
 in response to determining that the cooling demand ceases to be below the threshold cooling demand, de-energizing the VRF system, thereby ceasing cooling operation of the VRF system. 
 
     
     
       15. The method of  claim 9 , further comprising, in response to detecting a change in the cooling demand:
 varying a degree of openness of the first expansion valve. 
 
     
     
       16. A variable refrigerant flow (VRF) system, the system comprising:
 a compressor operable to compress refrigerant; 
 a first valve; 
 a first expansion valve; 
 a second valve; 
 a second expansion valve; and 
 a controller, wherein, in response to determining that a cooling demand of the VRF system is below a threshold cooling demand, the controller is operable to generate an artificial load by:
 operating the first valve to direct refrigerant discharged from the compressor only into a first coil; 
 operating the first expansion valve to direct an amount of the refrigerant received from the first coil to the second expansion valve; 
 operating the second expansion valve to direct the amount of the refrigerant to a third coil; and 
 operating the second valve to direct the amount of refrigerant to flow from the third coil to the compressor. 
 
 
     
     
       17. The VRF system of  claim 16 , wherein the first and third coils are placed outside a building. 
     
     
       18. The VRF system of  claim 16 , further comprising a variable speed drive coupled to the compressor, wherein the controller is further operable to regulate a speed of the compressor by operating the variable speed drive. 
     
     
       19. The VRF system of  claim 16 , wherein the controller is further operable to operate one or more fans to blow ambient air across the first coil or the third coil and to regulate a speed of the one or more fans based in part on the cooling demand. 
     
     
       20. The VRF system of  claim 16 , wherein the first coil is configured as a condenser and the third coil is configured as an evaporator.